The application of multiple layers of thermoplastic compounds onto an electrical conductor is well known within the industry. Several methods and devices for applying multiple layers of insulation to an electrical conductor are found in U.S. Pat. Nos. 4,303,734; 3,947,173; 3,903,233; 3,820,927; 3,222,721; 4,081,232; 3,581,343; 3,737,490; and 4,247,504.
U.S. Pat. No. 4,303,734 teaches the multiple layer extrusion of thermoplastics onto an electrical conductor with the outer layer being applied at a point outside the extruder head. U.S. Pat. Nos. 3,947,173 and 3,903,233 teach a method and an apparatus for the extrusion of concentric thermoplastic sheaths about an advancing filamentary core. The method and apparatus disclosed in U.S. Pat. Nos. 3,947,173 and 3,903,233 are adapted for the dual extrusion of one layer of a foamed or expanded thermoplastic which is surrounded by a non-foamed layer of a similar thermoplastic. Rotating core tooling is disclosed in U.S. Pat. No. 3,820,927 which provide dual extrusion apparatus whereby the material that is being applied as the inner layer can be applied as the outer layer and vice versa by rotating the core or tooling 180.degree. within the crosshead. U.S. Pat. No. 3,222,721 teaches the application of two different colors of a similar thermoplastic over an electrical conductor as well as the application of a thermoplastic having protective properties over a more vulnerable thermoplastic by a dual extrusion process.
A fixed center extrusion die assembly for the application of a plurality of concentric layers of thermoplastic coating material over an advancing electrical conductor core is disclosed in U.S. Pat. No. 4,081,232. Apparatus for producing an extruded single layer tube including a centering device to guide an electrical conductor through an extrusion head and drawing the tube onto the conductor are shown in U.S. Pat. No. 3,581,343. U.S. Pat. No. 3,737,490 teaches the apparatus of a simple process for applying two similar thermoplastics and the continuous inline curing thereof. U.S. Pat. No. 4,247,504 teaches a method of using two separate crossheads to apply a plurality of coatings on an electrical conductor, with the second crosshead extruding two layers of equivalent material.
Applying multiple layers of different thermoplastic compounds to an electrical conductor allows desired physical, chemical and electrical properties of the different thermoplastics to be combined so that an optimum wire construction is achieved. An example of such a combination would be an inner layer of a compound that has superior electrical insulating properties and an outer layer that provides superior abrasion resistance or resistance to petroleum products.
The foregoing references relate to thermoplastic extrusion for coating electrical conductors or similar articles. Other methods of and apparatus for extruding thermoplastic materials through a crosshead are disclosed in U.S. Pat. Nos. 2,610,607; 2,521,123; 3,241,186; 4,056,344; and 3,461,197. It should be readily seen from the foregoing that the simultaneous application of two thermoplastic compositions to an electrical conductor is not unknown in the art and is in fact currently being practiced in the industry. It should also be evident from a reading of the above references that the successful application of a plurality of layers of thermoplastic insulating shielding has been accomplished when the thermoplastics have been of similar compositions or when the crossheads holding the extrusion tooling are remote from each other.
Many times it is desirable to apply dissimilar thermoplastic compounds which have very different melting temperatures to an electrical conductor. Generally, one of the two compounds has a much higher melting temperature than the other. The traditional method of accomplishing this multi-layered coating has been to apply the inner layer in one operation, and then to apply the outer layer in a separate step. This method of forming a multi-layered coating has several disadvantages, one of which is the cost of the extra processing steps. Another disadvantage of such a process is the increased possibility of damage to the product because of the additional handling inherent in the extra processing steps. Still another disadvantage is the extra time needed to carry out another processing step.
The application of a plurality of layers of dissimilar thermoplastics through a single segmented crosshead eliminates many of the disadvantages discussed above. One disadvantage eliminated is the extra processing steps and the additional costs associated therewith. Another disadvantage eliminated by the method and apparatus of the present invention is that of product damage which results from the extra processing step thereby reducing product cost by reducing scrap. Elimination of an extra processing step also significantly decreases processing time required to manufacture the product which further reduces product cost.
Attempts have been made to apply multi-layered coatings of thermoplastics to electrical conductors in a single crosshead. These previous attempts have used methods and apparatus which were incapable of sustained operation, particularly when it is desired to coextrude thermoplastics with substantially different melting points. In these early attempts, the segment of the head through which the higher melting point thermoplastic flows became a source of unwanted heat for the portion of the head used to apply the lower melting point compound making control of head and melt temperatures difficult. Because systems were unable to control head and melt temperatures, the cooler portion of the head (that through which the lower melting thermoplastic compound passes) is heated by the hotter portion of the head (that through which the higher melting thermoplastic passes) thereby causing the lower melting thermoplastic to overheat. When a thermoplastic being extruded overheats, it begins to undergo thermal degradation which causes hard particles to form in the extrudate with resulting decrease in surface quality and electrical properties of the insulated product.
Attempts to overcome this problem have typically been made by trying to separate the head segments from each other with some type of heat insulating material such as asbestos. A typical apparatus is disclosed in U.S. Pat. No. 3,538,547. These solutions have failed because they did not sufficiently restrict the heat path between the individual head segments of the crosshead and the cooler segment continued to be overheated by the hotter segment. Such an apparatus only slows the rate at which the cooler segment reaches a temperature at which thermal degradation of the lower melting thermoplastic begins and as a result, when this temperature is reached and degradation occurs, the production process must be stopped and the burned thermoplastic compound removed from the tooling before production can be continued.